A wearable computer that can be worn on a human body has attracted much attention, due to downsizing and sophisticating of mobile terminals. There has since been proposed a method as an example of data communications between such wearable computers, in which an electric field communication transceiver is integrally connected to a computer and allows the electric field that has been induced by the transceiver to transmit through a living body as an electric field transmission medium so as to carry out transmission/reception of data (for example, Japanese Patent Application Laid-open Publication No. 2004-153708 and United States Patent Application Publication 2004/009226).
FIG. 1 illustrates a circuit model including a living body, a transmission device (transmitting section) and a transceiver according to related art. A transmission circuit 105 modulates data to be transmitted that is outputted from an I/O circuit 102 by a prescribed frequency f in a modulation circuit 115 and thereby outputs the data. The transmission circuit 105 is off an earth ground 116 and parasitic capacitance 109 is caused between a ground 108 of the transmission circuit and the earth ground 116. By the way, Rs 113 is an output resistor of the transmission circuit 105.
In addition, there are caused parasitic capacitance 107 between a ground 108 of the transmission circuit 105 and a living body 104 and parasitic capacitance 110 between the living body 104 and the earth ground 116. The living body 104 and a mobile terminal 100 are connected with each other via a transmission electrode 111 and an insulator 112. In order to increase a voltage to be applied to the living body by causing resonance with the parasitic capacitances, a reactance section 106 is inserted between the transmission circuit and a transmission/reception electrode. In an electric field communication transceiver for use in electric field communication that is floating from the earth ground, there is known reactance adjustment that adjusts reactance of a variable reactance that has been inserted between the transmission/reception electrode and the transmission circuit by means of an amplitude monitor and a control signal generator in order to efficiently induce an electric field in a living body even when the parasitic capacitances are fluctuated (See the above-mentioned patent documents).
When such a circuit illustrated in FIG. 1 is used, a voltage amplitude |Vb| to be applied to the living body at the time of resonance is expressed by the following equation:
                                                    V            b                                    =                              1                          2              ⁢                              ΠR                s                            ⁢                              {                                                      C                    b                                    +                                                            C                      sb                                        ⁡                                          (                                              1                        +                                                                              C                            b                                                    /                                                      C                            g                                                                                              )                                                                      }                                              ⁢                                                V              s                                                                      (        14        )            where Rs represents an output resistance of the transmission circuit and |Vs| represents an amplitude of an output signal from the transmission circuit. In addition, the parasitic capacitances 107, 109, 110 are designated by Csb, Cg, and Cb, respectively.
When the transceiver 101 or the transmission device is thinned in order to down-size the mobile terminal 100, Csb is increased, thereby reducing the voltage amplitude |Vb| that can be to be applied to the living body in accordance with the equation (14). Therefore, sufficient voltage cannot be obtained in a down-sized transceiver or transmission device, which makes it difficult to carry out communications.
FIG. 2 is a schematic diagram where a variable reactance is employed. FIG. 2 illustrates an insulator 133 that comes in contact with a living body 131 such as a human body, a transmission/reception electrode 132 insulated by the insulator 133, and an I/O circuit 30 that exchanges data with an external information processing apparatus (not shown).
In addition, FIG. 2 illustrates a transmission circuit 134, a switch 135, a variable reactance section 136, an electric field detection optical section 137, a signal processing section 138, a switch 139, a demodulation circuit 140, a wave forming section 141, an amplitude monitor section 142, and a control signal generation section 143 as a configuration for transmitting and receiving data.
FIG. 3 illustrates a configuration of a variable capacitance reactance as an example of a variable reactance.
In FIG. 3, a variable capacitance reactance section 601 is provided with alternating signal terminals 609, 610, an inductor 687, a buffer amplifier 686, a variable capacitance diode 671 such as a varicap diode or the like, capacitors 685, 690, resistors 688, 691. The variable capacitance diode 671 and the inductor 687 compose a resonance circuit and electrostatic capacitance of the variable capacitance diode 671 is varied by a control signal 611 inputted from the control signal generation section 143, thereby enabling adjustment of a resonance frequency. By the way, since there is a limit to a voltage applicable (withstand voltage), the variable capacitance diode 671 has to be used in a voltage range not exceeding the withstand voltage.
In addition, in an electric field communication transceiver employing a variable reactance, it is difficult to adjust a reactance value to a best-suited or substantially best-suited value unless the amplitude monitor section and the control signal generator are used. When accompanying the amplitude monitor section and the control signal generator, a circuit dimension of the electric field communication transceiver becomes larger, which is inconvenient in terms of integration into a wearable computer. Additionally, it is also inconvenient because power consumption may be increased.
Regarding power consumption, the following problem may arise. For example, an electric field communication transceiver may be applicable to entering/leaving management in a certain building or room. In this case, when the mobile terminal operates on a battery, such a mobile terminal is inconvenient and less secured because a person carrying such a mobile terminal cannot come out of the room if the battery becomes dead after he or she has entered the room.
In order to circumvent such inconveniency, there is required a mechanism in which electric power is provided to the mobile terminal (electric field communication transceiver) in a designated place to activate the mobile terminal and transmit data. If this is implemented in electric field communication, users can open a gate only by touching a part of the gate without taking the mobile terminal such as an ID card or the like out of their pocket, which can improve convenience.
FIG. 5 illustrates a system in which the transceiver illustrated in FIG. 4 is used as an installed terminal side transceiver to which electric power is supplied. In a transceiver 701 as illustrated in FIG. 4, a ground 711 of a transmission circuit 703 that modulates data to be transmitted by a predetermined frequency f and outputs the modulated data is apart away from an earth ground 702, thereby causing parasitic capacitance Cg 704 therebetween.
In addition, there is caused parasitic capacitance Csb 704 between the ground 711 of the transmission circuit 703 and a living body 700 and parasitic capacitance Cb 705 between the living body 700 and the earth ground 702. In order to increase a voltage applied to the living body by causing resonance with these parasitic capacitances, a reactance section 710 is inserted between the transmission circuit 703 and a transmission/reception electrode 713.
FIG. 5 is a schematic view of a system enabling an electric power transmission employing the transceiver 701 of FIG. 4. In FIG. 5, Cgs 726 represents parasitic capacitance between the transmission/reception electrode 727 and an earth ground 730; Cb 723 represents parasitic capacitance between the living body and the earth ground; Cg 722 represents parasitic capacitance between a ground 725 of the mobile terminal side transceiver 716 and the earth ground 730; and ZL 718 (ZL=RL+XL) represents impedance of the mobile terminal side transceiver 716.
When Csg 724 and Cb 723 are so small as to be neglected, series resonance is caused by a reactance Xv 719, Cg 722, and ZL 718, thereby applying a higher voltage to a resistance component Re [ZL]=RL of ZL 718 to which electric power is supplied. However, Csg 724 and Cb 723 are rather large in fact and not neglected, it is difficult to apply a higher voltage to RL.
The present invention has been made in view of the above disadvantages and the objective thereof is, first of all, a provision of a transmission device and a transceiver that are capable of preventing a reduction in amplitude of transmission voltage due to an increase in parasitic capacitance between the transmission/reception electrode and the living body that is prompted by down-sizing the transceiver or transmission device, preventing a reduction in voltage to be applied to an electric field transmission medium, and improving electric communication quality.
In addition, another objective of the present invention is a provision of an electric field transceiver that is capable of improving a withstand voltage characteristic of a variable capacitance diode, thereby preventing resonance suppression arising from an electric characteristic of a variable capacitance diode, and realizing an electric field communication with sufficient intensity.
Moreover, yet another objective of the present invention is a provision of an electric field communication transceiver in which there is realized a variable reactance means capable of self-compensating without any compensation circuit for a reactance value, thereby realizing improved communication at a lower consumption of electric power with a compact circuit.
Furthermore, the objective of the present invention is a provision of an electric field communication transceiver and an electric field communication system that are capable of applying a higher voltage from an installation terminal side transceiver to a mobile terminal side transceiver, thereby supplying electric power to the mobile terminal side transceiver.